Solid lubricants are generally added to an engine oil in order to improve its lubricating performance. For example, Patent Document 1 disclosed a lubricating treatment for engines using a composition containing a solid lubricant and an organic medium.
Specific examples of solid lubricants are described in Patent Document 1, including molybdenum disulfide, tungsten disulfide, graphite, polytetrafluoroethylene, boron nitride, soft metals (e.g., silver, lead, nickel, copper), cerium fluoride, zinc oxide, silver sulfate, cadmium iodide, lead iodide, barium fluoride, tin sulfide, zinc phosphate, zinc sulfide, mica, boron nitrate, boric acid, fluorocarbon, zinc phosphide and boron.
In order to disperse such a solid lubricant, e.g. a solid lubricant containing a mixture of molybdenum disulfide, copper, silver and lead, in an engine oil, first the solid lubricant is ball milled into fine powder in the air or in the vacuum. Subsequently the finely-milled mixture is ball-milled in a vegetable oil (canola oil), to which an emulsifier is then added, to add to the engine oil.
Adding solid lubricants to an engine oil in this manner results in an enhancement of lubricating performance of the engine oil, allowing engine torque and horsepower to be improved.